This invention relates to piezoelectric devices, in particular to piezoelectric coaxial cables and to a method of preparing such devices.
A number of piezoelectric devices have hitherto been proposed, generally comprising two conductors separated by a piezoelectric material. Such devices have often been proposed for use as transducers since, when they are subjected to an applied pressure, for example caused by the impact of an object, or to acoustic pressure changes, a potential difference will be generated between the conductors by the piezoelectric material. Applications for such devices are numerous and include underwater hydrophones, instrusion detectors, strain transducers, and vibration sensors. A common configuration for such devices is a coaxial cable, comprising an inner conductor, an intermediate insulating member of piezoelectric material surrounding the inner conductor, and an outer conductor surrounding the intermediate member.
In recent years certain polymeric materials for example poly(vinylidene fluoride) (PVF.sub.2)and poly(vinylidene fluoride) copolymers have been suggested for use as piezoelectric materials. In order to maximize the piezoelectric properties of a vinylidene fluoride polymer, it is necessary to orient the polymer by stretching it, preferably up to its "natural" draw ratio of about 4:1 or beyond, in order to convert at least a portion of the polymer from its initial alpha or form II crystalline phase into its beta or form I crystalline phase. Simultaneously with, or subsequent to, the stretching operation, it is necessary to polarize the polymer by applying a high electric field across the polymer in a direction perpendicular to the direction of orientation in order to align the dipoles of the polymer. Electric field gradients of from 5 to 200 MV/m are typical for the polarizing operation, the maximum applied field gradient usually being determined by the dielectric breakdown strength of the polymer material. The step of polarizing the polymer is frequently referred to as "poling".
In the case of a piezoelectric coaxial cable, in order to maximize its piezoelectric response, the intermediate piezoelectric member would need to be stretched axially and polarized radially between an inner, central electrode or conductor and an outer electrode or conductor in order to convert it from an ordinary dielectric into a piezoelectric material. While the outer electrode may be applied to the intermediate layer after streching, or, if a corona poling method is employed, the cable may be passed through a corona discharge electrode and an outer conductor for the cable be subsequently provided, significant problems are encountered in the provision of an inner electrode for the cable. It is not possible to extrude the intermediate member onto a conventional metal conductor, e.g., a copper conductor, in that it would then be impossible subsequently to stretch the intermediate layer in order to convert it into the beta-phase. This problem is particularly acute when attempting to make long lengths of piezoelectric coaxial cable.
One solution is disclosed in U.K. Patent Application No. 2,055,018 (Obata et al, assigned to Kureha Kagaku Kogyo), in which a tube of piezoelectric polymer is filled with a low melting point material, for example a low melting point metal alloy. The difficulty presented by the incompatibility of ordinary metallic inner conductors with the process of stretching the piezoelectric polymer is avoided because stretching can be performed at about or above the melting point of the alloy, but still below the melting point of the piezoelectric polymer. However, such alloys are relatively brittle, especially after multiple melting-recrystallization cycles, causing piezoelectric coaxial cables made with low melting point alloy cores to be susceptible to a loss of electrical continuity due to breaks in the alloy. Furthermore, during the stretching process the alloy is molten and the piezoelectric member stretches as a free tube, resulting in a tendency to produce voids in the alloy which are undesirable because they cause poor electrical contact. Consequently, it is difficult to make long lengths of such coaxial cables.
Another solution is to prepare separately a tape of the piezoelectric polymer, stretch it, pole it, and then wrap it around the inner conductor. See, for example, U.S. Pat. No. 3,798,474 (Cassand et al., assigned to Institut Francais du Petroles, des Carburants et Lubrifiants) and U.K. Patent Application No. 2,042,256 (Quilliam, assigned to The Marconi Company). However, this process is disadvantageous in that it requires extra steps and results in poor electrical contact between the piezoelectric polymer and the inner conductor.
U.S. Pat. No. 4,303,733 to Bulle discloses filaments which are essentially coaxial cables comprising at least three layers, at least two of which are electrically conductive with at least one electrical insulating layer positioned between the two conductive layers. The patent discloses that the intermediate layer may be piezoelectric. It states that where the filament pursuant to the invention is to be provided with piezoelectric characteristics, the core component preferably is compressible, which is achieved either by utilizing hollow filaments or by selection of appropriate synthetic polymers, as for the example, polyolefins with low molecular weight or polyethers. The patent continues to say that a suitable form of execution consists of using as the core component, an electrically conductive, highly viscous liquid with metal and/or carbon black and/or graphite particles dispersed therein. Suitable highly viscous liquids mentioned are, e.g., cis- and transpolyacetylene of relatively low molecular weight.
Therefore, it can be advantageous to have a cable in which the piezoelectric member can be applied directly onto the inner conductor, preferably by extrusion, and stretched and polarized to render it piezoelectric, which also has low resistivity electrodes, and which can be readily produced in long lengths.